1. Field of the Invention
The present invention relates to a radio-frequency wireless circuit and more specifically to a frequency converter for use with low supply voltages.
2. Description of the Related Art
Up to now, much progress has been made in microstructuring semiconductor devices including transistors. In particular, the gate length of transistors used in CMOS (complementary metal oxide semiconductor) devices has been shrunk to less than 0.1 μm. MOS transistors having a gate length of this order have a frequency characteristic which exhibits a very fast response and are therefore very useful devices for implementing next-generation radio-frequency communications systems.
As for bipolar transistors, a SiGe heterojunction-based transistor has been developed, which exhibits a fast response and high drivability and is also promising for next-generation radio-frequency communications systems.
These fast devices however attain high-speed performance at the cost of their ability to withstand high voltages. For example, with MOS transistors, reducing the gate length results in an improvement in high-speed performance but is accompanied by a reduction in the thickness of the gate oxide, thus degrading their breakdown characteristics. With bipolar transistors, on the other hand, reducing the thickness of the base region results in an improvement in their high-speed performance but accompanied by a degradation in their breakdown characteristics. With both MOS transistors and SiGe bipolar transistors, the breakdown voltage of devices that can realize high-frequency characteristics is lowered to values of the order of 1 to 2 V. Devices using circuit configurations adapted for a supply voltage of 3 V and hitherto used in radio communications systems may therefore be short of resistance-to-voltage capability. For this reason, the power consumption of the devices will increase. It is desirable that the power consumption of circuits particularly used in portable wireless terminals be as low as possible; otherwise, the conversation time of the terminals will be reduced considerably.
As described above, where use is made of high-speed transistors having low withstand voltages, it is required to set the circuit supply voltage lower than the withstand voltages of the transistors. To this end, it is required to devise a circuit configuration which is different from conventional ones and can be operated from a low supply voltage. In addition, to adapt to portable wireless terminals, it is necessary to reduce the power consumption of circuits used therein.